1. Field of Invention
This invention relates to fabrication of film resistors having improved trimming properties and operating characteristics and while it is described with particular reference to thick film resistors, the invention is not so restricted and also may be used in the fabrication of thin film resistor networks.
More specifically, the invention relates to film resistors having new and useful horizontal geometry and their method of manufacture to provide film resistor networks which possess higher trim ratios for resistors of given dimensions, allow greater throughput and higher yields, and produce film resistor networks which possess superior operating characteristics after trimming.
2. Background Problem
FIG. 1 is a schematic illustration of the horizontal geometry of a film resistor network which is used substantially throughout the film resistor industry as evidenced by the teachings of U.S. Pat. No. 3,573,703 and U.S. Pat. No. 3,947,801, for example. As illustrated in FIG. 1, the effective length of the film resistor shown as 11 is identified by the letter l, the width of the film resistor by the letter w, the depth of a laser trimming cut 13 by the letter d, and the centering of the laser cut 13 by the letter c. The termination region comprised by electrode conductors 12A and 12B overlap a portion of the film resistor as shown by dotted lines 14A and 14B to insure a good conductor to resistor interface and these overlap regions are essentially a conductive region and do not enter into the resistance calculations. In reality, the overlap regions do affect the resistance value, but such effect is outside the scope of this disclosure and does not negate any of the following considerations. For the purpose of the following disclosure, the interface between the non-overlapped areas of film resistor 11 with the overlap regions as shown at 14A and 14B shall be defined as the side edges of the resistor film, the lower edge 14C where the laser cut or notch 13 is formed shall be considered the bottom and the upper edge 140 shall be considered to be the top region of the resistor film. The substrate upon which such resistor films normally are formed has not been shown merely for the purpose of simplifying the illustration.
For any given sheet resistivity of a film resistor such as shown at 11, the resistance is determined essentially by the length divided by the width (R=l/w). This statement, however, defines a boundary condition which applies only when l is much larger than w or when the width of the notch d is equal to the length l. The latter case does not occur in the instant disclosure because the resistor films herein described are trimmed by laser beam cutting and the width of the laser beam cut or notch is approximately 0.002 inches (2 milli-inches).
The integrated circuit (IC) products which were first introduced on a commercial scale in the electronics industry in the early 1960s developed from small scale integration (SSI&gt;10 circuits) to medium scale integration (MSI&gt;50 circuits) to the more recently introduced large scale integration (LSI&gt;100 circuits) techniques. These semiconductor IC circuits have been produced by the tens of billions and although the functions and circuit applications continually change there has been standardization of packaging of such circuits. The so called dual-inline-package (DIP) or originally the TO-116 package is so firmly entrenched in the electronics industry that all major manufacturer's packages have been made to be physically interchangeable in addition to being made compatible with the same automatic insertion equipment.
The passive components manufacturers (resistors, capacitors, inductors, etc.) classically have been producing discrete devices for use in electronic circuits. During the past six years, however, there has been emphasis on replacing discrete components with integrated circuit devices particularly where the application is iterative in nature. The name which has become prevalent for integrated passive components is "networks". For such IC passive components, such as resistor networks, the types of circuits and of course their component values vary but the package in which the networks are sold and used have to be standardized for interchangeability as mentioned above. At the present time there is increasing emphasis on the implementation of the dual-inline package (DIP) and the single-inline-package (SIP). The SIP also has been standardized in terms of lead spacing (0.100 inches) and height above the circuit board namely 0.350, 0.250 and 0.200 inches. The latter dimensioning has given ground to less than 0.185 inches to be compatible with the maximum DIP height of 0.185 inches to provide interchangeability between SIPs and DIPs.
The above-discussed miniaturized packages along with standard circuits, power, voltage, temperature coefficient of resistance, laser trimming techniques, etc. have placed constraints on the network design, network layout (horizontal geometry) and materials. To meet these constraints, and yet provide reliable operationsl resistor networks which can be economically produced, the present invention was devised.